I looked at various Yankee forums and the general feeling is the same. Why don't they like synthetic?
For my own interest I found a nice level country lane with the grass verge running directly up to the tarmac. The grass was wet and the tarmac was damp. Full blooded starts in two wheel drive with the inside wheels on the grass promoted equel wheel spin both sides. There was no sign of serious ripping of the grass. Today, in the Peak District, we found lots of moorland sealed roads with frozen sections on the n/side. Normal amounts of power did not promote any wheel spin on the n/side rear wheel.
In other words, the Truetrac is doing exactly what it says on the tin, so the synthetic stays . The internals in the Truetrac are basically steel gears, so I cannot see what synthetic could do to damage it.
On the Yankee forums there were a number of people who said basically what I have i.e. the unit works, what's the problem?
I have emailed back to Eaton with some specific questions, but so far no answer.
Don't hold your breath.
Roger